1. Technical Field of the Invention
The present invention relates to the acylation of aromatic ethers or phenols, and, more especially, to the acylation of aromatic ethers in the para-position relative to an alkoxy radical borne by the aromatic ring nucleus.
In a preferred embodiment of the invention, acetyl chloride or acetic anhydride is condensed with an aromatic ether to prepare an alkoxyaromatic alkylketone.
2. Description of the Prior Art
A conventional process for the preparation of aromatic ketones is via a Friedel-Crafts type acylation reaction.
The aromatic compound is reacted with an acylation or acylating agent in the presence of a catalyst, generally aluminum chloride.
Compare Kuroda et al, Sci. Papers Inst. Phys. Chem. Res., 18, pp 51-60 (1932), which describes the preparation of methoxyacetophenones by reacting an aromatic compound substituted by 1 to 3 methoxy groups with acetyl chloride in the presence of aluminum chloride.
The use of aluminum chloride, however, presents a number of drawbacks. Aluminum chloride is corrosive and an irritant. In addition, aluminum chloride must be used in large amounts, at least equal to the stoichiometric amount, due to complexation of the ketone formed. As a consequence, aluminum chloride is not a true catalyst.
At the end of the reaction, the aluminum chloride must be removed from the reaction medium via acidic or basic hydrolysis.
Such hydrolysis technique requires the addition of water to the reaction medium, considerably complicating the process, since the metallic cation, in particular the aluminum cation, forms polyoxo- and/or polyhydroxo-aluminum complexes of a milky consistency in the presence of water, and are difficult to separate. A lengthy and expensive treatment, thus, has to be carried out after hydrolysis, comprising extraction of the organic phase, separation of the aqueous and organic phases, and even drying of the latter. Hence, separation of the aluminum chloride is a long and expensive process.
Further, there is the problem with the aqueous saline effluents, which must be neutralized, mandating a supplemental operation.
Still further, the aluminum chloride cannot be recycled, since it has been hydrolyzed.
To overcome this problem, Atsushi Kawada et al, J. Chem. Soc. Chem. Commun., p 1158 (1993), describe carrying out the acylation of an aromatic compound by means of acetic anhydride, in the presence of a catalytic amount of a trifluoromethane sulfonate of a lanthanide, in particular ytterbium. However, the catalyst is expensive and difficult to synthesize.